Learn Spanish (Latin America): El chico es tranquilo e introvertido.

Usually, the Spanish word for "and" is y. However, when the word right after it starts with an "ee" sound (spelled with i- or hi-), the y changes to e. Because introvertido starts with i, we use e to prevent the two "ee" sounds from blending together.

Does the e sound different from y?

Yes! The regular "and" (y) sounds like the "ee" in "meet". The changed "and" (e) sounds like the short "e" in "pet" or "step".

Why use es here? Isn't being calm a temporary state that requires estar?

It depends on what you mean! If you meant the boy is feeling calm right now, you would use está tranquilo. But by using es (from the verb ser), this sentence describes his personality. It means he is a naturally calm person.

If we were talking about a girl, would the e change?

The e would stay exactly the same, but the nouns and adjectives would change to feminine. You would say, La chica es tranquila e introvertida.